Conventional fish hatchery management encompasses not only the conventional "hatchery", with its troughs and raceways, but also includes aquaculture systems previously considered inappropriate for rearing large numbers of fish in a captive environment. The selection of a proper dissolved oxygen concentration range for a particular aquaculture system is but one of many variables which is deemed desirable to control, for optimizing fish hatchery operations.
The maintenance of adequate dissolved oxygen levels in aquaculture systems is a complex function dependent upon the different types or species of fish which inhabit the aquaculture system. For example, it is necessary not only to consider the relative population densities of the different species of fish, and their individual sizes and general health, it is also necessary to consider the population densities of various secondary organisms such as zooplankton, phytoplankton and algae, as well. Moreover, it is known that phytoplankton, commonly found in catfish ponds, consume oxygen during periods of low solar radiation. The phytoplankton may deplete the dissolved oxygen in the pond to a point where the catfish suffer from anoxia. During such periods, it is desirable to be able to oxygenate the pond to insure survival of the catfish during this period. The ability of the aquaculture system to sustain desirable fish development is also affected by water conditions such as temperature and pH, as well as carbon dioxide, nitrogen and ammonia content of the water. For example, dissolved oxygen concentrations of aquaculture water systems are depleted of their oxygen content in a variety of ways. Major oxygen-depletion mechanisms include respiration of fish and other organisms, and chemical reaction with organic matter such as feces, and decaying plant and animal matter.
It is well known that gases such as oxygen, nitrogen and carbon dioxide are each relatively more soluble in colder water than they are in warmer water. Considering only the respiration of fish, however, metabolism increases as water temperature increases, thereby necessitating increased amounts of oxygen to sustain healthy fish growth. Also, nitrogen and/or carbon dioxide are known to displace oxygen. Excessive concentrations of nitrogen or carbon dioxide, moreover, can be harmful to fish.
Thus, because adequate amounts of dissolved oxygen are deemed critical for desirable fish growth and survival, the maintenance of predetermined dissolved oxygen concentrations is of major concern to fish culturists. In general, fish do well at dissolved oxygen concentrations above about 4 parts per million (p.p.m.). In particular, a dissolved oxygen concentration of about 5 p.p.m. oxygen is preferred, and a dissolved oxygen concentration of slightly more than about 5 p.p.m. oxygen is even more preferred.
Excessive levels of oxygen concentration, however, may induce emphysema in fish, which again is undesirable. Generally, however, oxygen-related problems in fish are caused by gas concentrations that are too low. Fish have been known to survive extended periods (i.e. days) at about 3 p.p.m., but generally do not grow well. Most fish can tolerate about 1 to about 2 p.p.m. for only a few hours, and will die if oxygen concentrations at this level are extant for a prolonged time period or drop below this level.
Recent studies have suggested that it is desirable, for the purpose of optimizing fish production, to maintain dissolved oxygen concentrations in the aquaculture system water at about the oxygen saturation level therefor at the ambient conditions thereof. (In general, the term "saturation" refers to the amount of a gas that is dissolved in a known quantity of water when the water and atmospheric phases of the dissolved gas are in equilibrium.)
In ponds that have no flowing fresh water supply, oxygen comes from only two sources, namely, diffusion from the air, and photosynthesis. Oxygen diffuses across the water surface into or out of the pond, depending on whether the water is subsaturated or supersaturated with respect to the gas. Once oxygen from the air enters the surface film of the water, it diffuses relatively slowly through the bulk of the water mass. Generally, only if surface water is mechanically mixed with the rest of the pond--by, for example, wind, pumps, aeration devices, or outboard motors--will diffused oxygen be significant in aerating the entire body of water.
Unfortunately, the oxygen-transfer or oxygen-diffusion efficiency of conventional aeration devices--such as diffused air aerators or mechanical surface aerators--is low, due principally to the relatively shallow depth of most aquaculture systems.
The requirement of maintaining the substantially saturated dissolved-oxygen concentrations at ambient conditions presents an additional impediment in aquaculture systems because in many such systems the phytoplankton, which are the major producers of oxygen when light induces photosynthesis, typically become the major consumers of oxygen in the absence of photosynthesis.
Especially during the fish growing season, the dissolved oxygen concentration in any aquaculture system is determined primarily by the balance of photosynthesis and respiration. For fish culture to flourish, dissolved oxygen concentration ranges of an aquaculture system must be controlled within predetermined limits that are dependent upon the particular type of fish culture being reared. For example, the lowest safe level for trout is about 5 p.p.m. dissolved oxygen. Other fish species, as mentioned above, may be able to tolerate a somewhat lesser dissolved oxygen concentration. However, the object is not to establish an oxygen concentration that fish can tolerate. It is desirable to be able to maintain the dissolved oxygen concentration at a fish-flourishing level that is optimal for the particular fish species that is being cultivated.